This invention relates to apparatus for compensating ship motion and sea-state conditions in order to protect oceanographic instrumentation suspended on a support cable that might otherwise part during use or recovery due to high shock loads in the support or instrumentation cable.
Recovery of advanced sonobuoys poses a problem for test and evaluation. During high sea-state conditions, the sonobuoy sensor is susceptible to loss due to high transient shock loads in the cable. A ship motion compensator is required that will greatly increase the probability of recovery for both "A" and "B" size sonobuoy stores.
The general trend in advanced sonobuoy design is to place the sensor very deep in the water column. This requirement to go deep is predicated on achieving greater acoustic range and higher probability of detection. With the need to go deep the amount of cable used places greater demands on the package volume. This in turn leads to cables of small diameter with minimal factors of safety.
Design consideration is always given to ensure that the forces on the sonobuoy cable, during both its deployment and operation phases, do not exceed the breaking strength. For test and evaluation purposes, the same consideration should be given during the recovery of the cable and sensor. Unfortunately, the recovery in many instances represents the worst case situation; a surface vessel is used and the cable is normally reeled directly on a deck winch. Under these conditions high snaploads are likely to occur.
It is axiomatic that test and evaluation is often required when the recovery conditions are at their worst, and this is often necessary so that the effect of high sea states on sonobuoy performance can be examined. To ensure reliable recovery of all deep sonobuoys, a means, passive or active, of eliminating these high shock loads is required.
Prior art techniques may be divided into two general methods of eliminating transient dynamic loads in suspension cables. These divisions are known as "active" and "passive" compensation methods. The "active" method reduces the dynamic loads by sensing the transient onset and adjusting the amount of cable payed out. Two examples of this type of compensation are: a constant tension winch, built by Pratt and Whitney for Scripps Institute of Oceanography, and a constant tension crane built by Ocean Systems Engineering for the Navy. This type of machinery is inherently complex, massive and very expensive. Its utility lies in the deployment and recovery of very large and heavy cables where the dynamic conditions vary over such a large range that the use of a "passive" compensator would be impossible.
A "passive" compensator relies on reducing the dynamic loads by "tuning" the suspension frequency below the excitation frequency. The compensator therefore becomes a low-rate spring inserted between the deck winch and the suspended cable. In the past, methods of implementing this low-rate spring have included the pneumatic/hydraulic ram and rubber bungee, as just noted.
Both of these methods leave a lot to be desired. To compensate for large motions (10 to 15 feet) the size and complexity of the pneumatic ram method becomes unattractive. The bungee rubber method provides a low cost approach. However, much of this advantage is offset by its tendency to deteriorate rapidly with usage and sunlight. Also, terminations made to the bungee are very unreliable and are likely to fail at the most inconvenient moment. To obtain the high strength and low spring rate, very long lengths of bungee are required. This in turn leads to very complex sheave arrangements.
Still another passive technique consists of the use of a spring-loaded drum. Typical systems using this alternative passive technique are disclosed in U.S. Pat. No. 3,020,567. Spiral springs have been used in other applications such as in tensioning a tagline in a crane mechanism, as shown in U.S. Pat. No. 2,367,912; retrieving electrical cable and the like as shown in U.S. Pat. No. 3,033,488; and in suspending a bucket or the like at the end of a hoist cable. Other applications of spiral-type tension springs in connection with cable retrieval are disclosed in U.S. Pat. Nos. 3,593,941 and 2,130,504. However, although spiral springs have been used in applications for providing constant tension, they have not been employed as motion compensators in a cable retrieval system. The advantage of so using a spiral spring resides in the ability to select a tension, by using the appropriate reel diameter for a given spring tension, and in the need to reel the cable through only one sheave in providing the desired tension compensation to reduce the fatigue stressing the cable fiber.